Observation of single-walled carbon nanotubes by photoemission microscopy

Single-walled carbon nanotube networks grown on SiO₂ pillars were studied by means of scanning photoemission microscopy. The individual nanotubes or nanotube bundles growing from the pillar tops were observed in C 1s images. Band bending near catalytic Fe/nanotube contacts in an end-bonded configuration was studied by measuring C 1s spectra along the tube axes. Within our experimental resolution, no band bending was observed. This implies that the depletion width is less than the spatial resolution of the scanning photoemission microscope (90 nm) or that the amount of the band bending is less than 0.1 eV.     

Theory of chromatography of complex cyclic polymers: eight-shaped and daisy-like macromolecules

A theory of chromatography of eight-shaped, trefoil-shaped and daisy-like polymers is developed. For a model of an ideal chain in a slit-like pore exact equations and a number of approximate formulae for the distribution coefficient K of these polymers are derived. All modes of chromatography of complex macrocycles of arbitrary molar mass in both narrow and wide pores are covered by the theory. It is shown that complex macrocycles always elute after linear polymers and rings of the same contour length. The effective chromatographic radius of eight-shaped and daisy-like macromolecules, which determines retention in size-exclusion chromatography are calculated. The increase in the retention with molar mass is predicted for all types of macrocycles at the critical interaction condition. Non-monotonous molar mass dependences of K are found at pre-critical interaction. We simulate separation of complex cyclic polymers from linear and ring precursors, discuss possibilities to separate symmetric and asymmetric eights, and speculates on the use of chromatography for separating knotted and unknotted polymer rings. According to the theory, the chromatography under the critical and pre-critical interaction conditions is expected to be especially efficient in these and similar problems. Boundary conditions for the theory and its applicability to real systems are discussed.     

In situ ESR of CrH-ZSM-5 up to 500°C in flowing gas containing O2, H2O,CCl4, NO,NO2, and C3H6

A flow cell was used for the in situ ESR monitoring of the state and reactivity of chromium ions in Cr-ZSM-5. Calcination of Cr(NO₃)₃/NH₄-ZSM-5 in air at 500°C is accompanied by migration of chromium ions inside the zeolitic channels and stabilization ofisolated Cr⁵⁺ cations near lattice A1³⁺ ions. Calcination of Cr-ZSM-5 at 750°C leads to a gradual disappearance of the isolated Cr⁵⁺ cations and formation of -Cr₂O₃ microcrystals. All the Cr⁵⁺ cations are accessible to gas-phase molecules: O₂ strongly broadens the dipole-dipole signal; H₂O sorption increases the local crystal field symmetry; admission of CCl₄ results in a small change of the Cr⁵⁺ local coordination; strongly stabilized complexes on Cr-ZSM-5 are observed upon sorption of either NO or NO₂. The sorption of C₂H₆ on Cr-ZSM-5 at 20°C is accompanied by a gradual reduction of the Cr⁵⁺ sites. At 500°C in [C₃H₆ + O₂ + He] flow, even at a large excess of oxidant, the reduction of a noticeable part of Cr⁵⁺ ions takes place. At 400°C, in the same gas mixtures, a deeper reduction of Cr⁵⁺ occurs. Closer to stoichiometric conditions, in a [C₃H₆ + NO + He] flow with 120% excess of oxidant the Cr⁵⁺ is completely reduced at 500°C. The oxidation of propene is accompanied by coke deposition on the surface of the catalyst. The implications of the results are discussed.     

Calcium Ions Regulate K+ Uptake into Brain Mitochondria: The Evidence for a Novel Potassium Channel

The mitochondrial response to changes of cytosolic calcium concentration has a strong impact on neuronal cell metabolism and viability. We observed that Ca²⁺ additions to isolated rat brain mitochondria induced in potassium ion containing media a mitochondrial membrane potential depolarization and an accompanying increase of mitochondrial respiration. These Ca²⁺ effects can be blocked by iberiotoxin and charybdotoxin, well known inhibitors of large conductance potassium channel (BK_Ca channel). Furthermore, NS1619 – a BK_Ca channel opener – induced potassium ion–specific effects on brain mitochondria similar to those induced by Ca²⁺. These findings suggest the presence of a calcium-activated, large conductance potassium channel (sensitive to charybdotoxin and NS1619), which was confirmed by reconstitution of the mitochondrial inner membrane into planar lipid bilayers. The conductance of the reconstituted channel was 265 pS under gradient (50/450 mM KCl) conditions. Its reversal potential was equal to 50 mV, which proved that the examined channel was cation-selective. We also observed immunoreactivity of anti-β₄ subunit (of the BK_Ca channel) antibodies with ~26 kDa proteins of rat brain mitochondria. Immunohistochemical analysis confirmed the predominant occurrence of β₄ subunit in neuronal mitochondria. We hypothesize that the mitochondrial BK_Ca channel represents a calcium sensor, which can contribute to neuronal signal transduction and survival.     

Theoretical limit of localized surface plasmon resonance sensitivity to local refractive index change and its comparison to conventional surface plasmon resonance sensor

In this paper, the theoretical sensitivity limit of the localized surface plasmon resonance (LSPR) to the surrounding dielectric environment is discussed. The presented theoretical analysis of the LSPR phenomenon is based on perturbation theory. Derived results can be further simplified assuming quasistatic limit. The developed theory shows that LSPR has a detection capability limit independent of the particle shape or arrangement. For a given structure, sensitivity is directly proportional to the resonance wavelength and depends on the fraction of the electromagnetic energy confined within the sensing volume. This fraction is always less than unity; therefore, one should not expect to find an optimized nanofeature geometry with a dramatic increase in sensitivity at a given wavelength. All theoretical results are supported by finite-difference time-domain calculations for gold nanoparticles of different geometries (rings, split rings, paired rings, and ring sandwiches). Numerical sensitivity calculations based on the shift of the extinction peak are in good agreement with values estimated by perturbation theory. Numerical analysis shows that, for thin (≤10 nm) analyte layers, sensitivity of the LSPR is comparable with a traditional surface plasmon resonance sensor and LSPR has the potential to be significantly less sensitive to temperature fluctuations.     

Coercivity and domain structure of nanograined Fe–C alloys after high-pressure torsion

The microstructure and magnetic properties of binary hypo- and hyper-eutectoid Fe–C alloys were studied. The investigations have been carried out on the samples in the as-cast state, after a long annealing at 725 °C and on the specimens after the high-pressure torsion (HPT). The deformation was carried out at the ambient temperature and the pressure of 5 GPa. The grain size after HPT is in the nanometer range. Long annealing leads to a drastic decrease of the coercivity in comparison with the as-cast alloys. In all alloys the coercivity H _c increases with increasing carbon content. The distance L between pinning points for domain walls decreases with increasing carbon content. Increase of the coercivity and decrease of L are more pronounced below the eutectoid concentration. The coercivity of the nanostructured samples is higher than that of the as-cast alloys. Due to the pinning of domain walls by the cementite particles, the hysteresis loop in the coarse-grained alloys both in as-cast and annealed states has a narrowing near the zero magnetization.     

Direct free-form deformation of NC programs for surface reconstruction and form-error compensation

In this paper a new approach for manufacturing modified workpieces by milling is presented. In course of product development, several optimization iterations are often required, in which the shape of the workpiece is modified. Conventional method for manufacturing modified workpieces includes a time-consuming and error-prone step of reverse engineering, where the new CAD/CAM data is generated with respect to the measurement data of the manufactured workpiece. The new approach generates a continuous deformation function in order to approximate the discrete displacement vectors between the initial and the modified shapes, and applies this function on the original NC programs of the workpiece. Hence, it is possible to directly manufacture the modified shape. The process of reverse engineering can be eliminated so that manufacturing costs and the time from workpiece design to the production decrease significantly.     

N2O Decomposition Catalysed by Transition Metal Ions

The ignition processes for the catalytic partial oxidation of methane (POM) to synthesis gas over oxidic nickel catalyst (NiO/Al₂O₃), reduced nickel catalyst (Ni⁰/Al₂O₃), and Pt-promoted oxidic nickel catalyst (Pt–NiO/Al₂O₃) were studied by the temperature-programmed surface reaction (TPSR) technique. The complete oxidation of methane usually took place on the NiO catalyst during the CH₄/O₂ reaction, even with a pre-reduced nickel catalyst, and Ni⁰ is inevitably first oxidized to NiO if the temperature is below the ignition temperature. It is above a certain temperature that Ni⁰ is formed again, which leads to the start of the POM. The POM can be initiated at a much lower temperature on a Pt–NiO catalyst because of Pt promotion of the reduction of NiO. The POM in a fluidized bed can be easily initiated due to the addition of Pt.     

Automated liquid-liquid extraction by pneumatic recirculation on a centrifugal microfluidic platform

In this technical note, a liquid-liquid extraction technique was performed using pneumatic liquid recirculation on a centrifugal microfluidic device. Non-contact pneumatic pumping enabled a multi-cycle liquid-liquid extraction process using aqueous iodine in a potassium iodide solution and hexadecane while requiring a minimal amount of space on the device. The extraction process was completely automated on the device following sample introduction and required only 50 s for each extraction cycle. The pumping rate achieved during liquid recirculation was 120 ± 10 μL/min. A recycling process such as the one demonstrated would be difficult to implement in a conventional centrifugal microfluidic system.     

Characterization of atmospheric aerosols from infrared measurements: simulations, testing, and applications

An inversion method for the characterization of atmospheric condensed phases from infrared (IR) spectra is described. The method is tested with both synthetic IR spectra and the spectra of particles that flow in a cryogenic flow tube. The method is applied to the IR spectra recorded by the Atmospheric Trace Molecule Spectroscopy instrument carried by the Space Shuttle during three missions in 1992, 1993, and 1994. The volume density and particle size distribution for sulfate aerosol are obtained as a function of altitude. The density and size distribution of ice particles in several cirrus clouds are also retrieved. The probable radius of the ice particles in the high-altitude (10-15-km) cirrus clouds is found to be approximately 6-7 microm.